=, ait: - 
March 9, 1882} 
7 ~ 
NATURE 439 
who have so fully studied these plants when cultivated on 
the same materials. For this purpose, it was necessary 
to obtain the oosporangia ; and in ordinary course, these 
should have made their appearance on my Safrolegnia in 
five or six days. Unfortunately, in the course of cultiva- 
tion continued over two months, nothing of the kind has 
taken place. Zoosporangia have abounded in the ordinary 
form and also in that known as “ dictyosporangia,” but, 
in no instance, have any oosporangia appeared. After a 
few days of vigorous growth, the zoosporangia become 
scanty, and the fungus takes on a torulose form, segments 
of the hyphz becoming swollen and thea detached as 
independent “gemme,” which may germinate. Sometimes 
the gemme are spheroidal and terminal, and closely 
simulate oosporangia. 
Although, therefore, 1 have very little doubt that the 
Saprolegnia of the salmon is one of the forms of the 
“¢ S. ferax group” of Pringsheim and De Bary, I have, at 
present, no proof of the fact. 
Another very curious and unexpected peculiarity of the 
salmon Saprodegnia, both on the fish and when transmitted 
to flies,so far as my observations have hitherto gone, is 
that locomotive ciliated zoospores do not occur. I once 
saw one which exhibited a very slight motion for a few 
minutes after it left the zoosporangium ; but although 
thousands must have passed under my notice, with the 
exception to which I have referred, they have always been 
perfectly quiescent and not unfrequently in different stages 
of germination. Whether the season of the year, or the 
conditions under which my saprolegnised flies were 
placed, have anything to do with the non-appearance of 
oosporangia and of locomotive zoospores in them I cannot 
say. But itis certain that the Safrolegnia ferax which 
commonly appears upon dead flies and other insects 
normally develops both oosporangia and locomotive 
zoospores in abundance. « 
From such notices by other observers as I can gather, 
oosporangia appear to be of very rare occurrence in the 
Saprolegnia of the salmon itself. Mr. Stirling mentions 
that he has met with them only four times. With respect 
to locomotive zoospores, I can find no positive evidence 
that they have been regularly, or even frequently, observed 
in the salmon Safrolegnia. But these points require 
careful investigation on freshly taken diseased fish. 
Whether the zoospores are actively locomotive or not, 
they are quite free when they emerge from the zoospo- 
rangia ; and, from their extreme minuteness, they must 
be readily carried away and diffused through the sur- 
rounding water. Hence, a salmon entering a stream 
inhabited by the Safrolegnia will be exposed to the 
chance of coming into contact with Saprolegnia spores ; 
and the probability of infection, other things being alike, 
will be in proportion to the quantity of the growing 
Saprolegnia, and the vigour with which the process of 
spore-formation is carried on, Ata very moderate estimate, 
a single fly may bear 1,000 fruiting hyphe ; and if each 
sporangium contains twenty zoospores, and runs through 
the whole course of its development in twelve hours, the 
result will be the production of 40,000 zoospores in a day, 
which is more than enough to furnish one zoospore to the 
cubic inch of twenty cubic feet of water. Even if we 
halve this rate of production, it is easy to see that the 
Saprolegnia ona single fly might furnish spores enough 
to render such a small shallow stream as salmon often 
ascend for spawning purposes, dangerous for several days. 
But a large fully diseased salmon may have as much as 
two square feet of its skin thickly covered with Safvo- 
Zegnia. If we allow only 1,000 fruiting hyphz for every 
square inch, we shall have 288,000 for the whole surface, 
which, at the same rate as before, gives over 10,000,000 
) spores for a day’s production, or enough to provide a 
spore to every cubic foot of a mass of water 100 feet wide 
and five feet deep and four miles long. Forty such 
diseased salmon might furnish one spore to the gallon 
for all the water of the Thames (380,000,000 gallons per 
diem) which flows over Tedington Weir. But two 
thousand diseased salmon have been taken out of a single 
comparatively insignificant river in the course of a 
season. 
It will be understood that the above numerical estimate 
of the productivity of Saprolegnia, has been adopted 
merely for the sake of illustration; that I do not intend 
to suggest that the zoospores are evenly distributed 
through the water into which they are discharged by the 
zoosporangia ; and that allowance must be made for the 
very short life of those zoospores which do not speedily 
reach an appropriate nidus. Nevertheless, the conclusion 
remains arithmetically certain that every diseased salmon 
adds immensely to the chances of infection of those which 
are not diseased ; and thus, the policy of extirpating every 
diseased fish as soon as possible, has ample justification. 
But, in practice, the attempt to stamp out the disease in 
this fashion would be so costly that it may be a question 
whether it is not better to put up with the loss caused by 
the malady. 
There are many practical difficulties in the way of 
directly observing the manner in which the zoospores 
effect their entrance into the skin of the fish; but, on 
comparing the structure of the healthy integument with 
that of the diseased patches, the manner of the operation 
can readily be divined. The skin of the head of a salmon, 
for example, presents a thin superficial cellular epidermis 
covering the deep fibrous and vascular derma. The 
epidermic cells are distinguishable, as in fishes in general, 
into a deep, a middle, and a superficial layer. In the 
first, the cells are vertically elongated, in the second 
more rounded and polygonal, in the third flattened. 
Many of the cells of the middle layer are of the nature of 
“mucous cells.’? They enlarge and become filled with a 
mucous secretion ; and, rising to the surface, burst and 
discharge their contents, which give rise to the mucous 
fluid with which the fish’s body iscovered. The openings 
of these “ mucous cells’’ remain patent for some time and 
are to be seen in thin vertical sections. The hyphze of 
the spores which attach themselves to the fish may enter 
by these openings, but even if they do not, the flattened - 
superficial cells certainly offer no greater resistance than 
does the tough cuticle of a fly. However this may be, 
sections of young patches of diseased skin show 
that the hyphz of the fungus not only traverse the 
epidermis, but bore through the superficial layer of the 
derma for a distance, in some cases, of as much as one- 
tenth of an inch. Each hypha thus comes to have astem- 
part, which lies in the epidermis, and a root-part, which 
lies inthe derma. Each of these elongates and branches 
out. The free ends of the stem-hyphe rise above the 
surface of the epidermis and become converted into 
zoosporangia, more or fewer of the spores of which attach 
themselves to the surrounding epidermis and repeat the 
process of penetration. Thus the epidermis and the derma 
become traversed by numerous hyphe set close side by 
side. But, at the same time, these hyphze send off lateral 
branches which spread radially, forcing asunder the middle 
and deeper layers of the epidermic cells, and giving rise 
to the radiating ridges which are visible to the naked eye 
in the peripheral part of the patch. The force of the 
growth of the hyphz which traverse the epidermis, is 
made obvious by the curious manner in which, when the 
central tract of a patch is teased out, the distorted 
epidermis cells are seen adhering to it as if they were 
spitted upon it. 
In the derma, the root-hyphz branch out, pierce the 
bundles of connective tissue, and usually end in curiously 
distorted extremities. 
The effect of the growth of the stem-hyphe is to destroy 
the epidermis altogether. Its place is taken by a thick, 
felted, mycelium, which entangles the minute particles of 
sand which are suspended in the water, and thus no doubt 
